1. Field of the Invention
This invention relates to the adsorptive separation of metaxylene from a complex admixture of aromatic hydrocarbons including other xylenes. The invention more specifically relates to the recovery of metaxylene from an orthoxylene-containing stream by selective liquid-phase adsorption using certain aluminosilicate zeolitic molecular sieves at specific hydration levels and temperatures.
2. Related Art
Those skilled in the art are familiar with a number of adsorptive separation methods which may be employed to perform the separation of aromatic hydrocarbons. The use of zeolitic adsorbents in these methods has been well described in the literature. For instance, U.S. Pat. No. 3,840,610 issued to J. A. Hedge is directed to the adsorptive separation of a number of organic compounds such as 2,6 dimethyl naphthalene. This reference also indicates a sodium Y zeolite will separate m-xylene from p-xylene.
U.S. Pat. No. 4,306,107 issued to D. B. Broughton describes the use of a sodium containing zeolite Y at a 2-7 wt. % water content by LOI to recover metaxylene from admixture with both other xylenes. Accompanying this disclosure is a discussion of the use of toluene as a desorbent, and the possibility of including silica, alumina or clay with the zeolite. This reference differs in operation as it appears limited to an adsorption process which removes two raffinate streams, with the intermediate raffinate being removed from the adsorption zone. The reference also does not describe any sensitivity of the separation to the water content of the adsorbent or the operating temperature.
U.S. Pat. No. 4,326,092 issued to R. W. Neuzil also describes the recovery of metaxylene from three isomer xylene admixtures through the use of a sodium exchanged Y zeolite. This reference describes suitable adsorbents in terms of silica to alumina ratios and exemplifies a temperature of 150.degree. C. The reference appears silent as to the influence of adsorbent hydration and operating temperature on adsorbent capacity and selectivity versus orthoxylene content. That is, this reference appears not to provide any teaching about operative LOI ranges and appears to operate only on feeds having a low concentration of ortho xylene.
U.S. Pat. No. 3,204,006 issued to D. B. Broughton describes an adsorptive separation process in which a potentially very broad composition feed stream is contacted with an adsorbent which recovers mono and polycyclic aromatics. This reference teaches many variations to this general step. In one the desorbed mixed aromatics are then charged to a hydrodealkylation zone, with the monocyclic component being used as the desorbent. The teaching of the patent thus appears limited to a bulk aromatic/paraffin separation compatible with its objectives such as improving diesel fuels and producing an aromatic product from a mixture comprising both aromatics and nonaromatics. The product aromatics may contain one or more short side chain groups. The patent does not appear to suggest or teach the recovery of specific aromatic isomers by adsorption, and in fact states the aromatic product may be fractionated by conventional means not illustrated to recover individual species.